Rechargeable batteries are used in many electronic devices, including very common usage in laptop and notebook computers and similar mobile computing devices. The batteries may include lithium ion (Li-ion) and other battery technologies. However, defects in the batteries can create harmful or dangerous conditions if the defects are not detected.
Among the possible defects are short circuits (commonly referred to as shorts) inside the batteries, including short circuits within Li-ion battery cells. A short circuit conducts current inside the battery cell, causing local heating and, under certain circumstances, thermal runaway. A subset of these shorts may exist as so-called “soft shorts” prior to the short circuits becoming severe enough to produce thermal runaway conditions.
When a battery pack is charged, the cell blocks (where each cell block may contain one or more battery cells) of the battery pack are commonly charged in a manner intended to equalize the charge in the cell blocks and thus generate the greatest electrical capacity for each cell block. In such a process, certain cell blocks may be provided with additional charge if such cell blocks are found to be low in charge in comparison with the other cell blocks in the battery pack. A battery management unit may bleed current around a cell block that has reached its fully charged state (the termination voltage) faster than other blocks, or may actively pump charge from one cell block to another, in order to balance the stored charge in each cell.
However, it is possible that a cell block that requires more charge in order to reach full charge may have a higher self-discharge rate due to the presence of a developing internal short in one or more battery cells of the cell block. Such a battery cell may be referred as a “weak” cell. Providing more charge to such weak cells to equalize charge does not take account of the possibility of a nascent short being the cause of the low charge state, and a battery management unit may thus risk the onset of potentially dangerous developments. For example, an internal short may discharge a battery cell below its discharge cutoff, thereby precipitating further issues, such as copper dissolution, that may further compromise the safety and reliability of the cell. Continuing to charge or discharge such a weak cell may cause a benign short to become a dangerous one.